The publication of the Darwin and Wallace papers in 1858, and still more that of the ‘Origin’ in 1859, had the effect upon them of the flash of light, which to a man who has lost himself in a dark night, suddenly reveals a road which, whether it takes him straight home or not, certainly goes his way. That which we werelooking for, and could not find, was a hypothesis respecting the origin of known organic forms, which assumed the operation of no causes but such as could be proved to be actually at work. We wanted, not to pin our faith to that or any other speculation, but to get hold of clear and definite conceptions which could be brought face to face with facts and have their validity tested. The ‘Origin’ provided us with the working hypothesis we sought.
“About thirty years ago there was much talk that geologists ought only to observe and not theorize; and I well remember someone saying that at this rate a man might as well go into a gravel-pit and count the stones and describe the colors…how odd it is that anyone would not see that all observation must be for or against some view, if it is to be of any service.”
Charles Darwin, letter to Fawcett, 18 September 1861
I’m a geologist by training, so you might see a few of these.